Re: [PATCH] drm/shme-helpers: Fix dma_buf_mmap forwarding bug

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 27 Oct 2020 22:49:22 +0100
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> When we forward an mmap to the dma_buf exporter, they get to own
> everything. Unfortunately drm_gem_mmap_obj() overwrote
> vma->vm_private_data after the driver callback, wreaking the
> exporter complete. This was noticed because vb2_common_vm_close blew
> up on mali gpu with panfrost after commit 26d3ac3cb04d
> ("drm/shmem-helpers: Redirect mmap for imported dma-buf").
> 
> Unfortunately drm_gem_mmap_obj also acquires a surplus reference that
> we need to drop in shmem helpers, which is a bit of a mislayer
> situation. Maybe the entire dma_buf_mmap forwarding should be pulled
> into core gem code.
> 
> Note that the only two other drivers which forward mmap in their own
> code (etnaviv and exynos) get this somewhat right by overwriting the
> gem mmap code. But they seem to still have the leak. This might be a
> good excuse to move these drivers over to shmem helpers completely.
> 
> Note to stable team: There's a trivial context conflict with
> d693def4fd1c ("drm: Remove obsolete GEM and PRIME callbacks from
> struct drm_driver"). I'm assuming it's clear where to put the first
> hunk, otherwise I can send a 5.9 version of this.
> 
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Lucas Stach <l.stach@pengutronix.de>
> Cc: Russell King <linux+etnaviv@armlinux.org.uk>
> Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
> Cc: Inki Dae <inki.dae@samsung.com>
> Cc: Joonyoung Shim <jy0922.shim@samsung.com>
> Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
> Cc: Kyungmin Park <kyungmin.park@samsung.com>
> Fixes: 26d3ac3cb04d ("drm/shmem-helpers: Redirect mmap for imported dma-buf")
> Cc: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Rob Herring <robh@kernel.org>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: <stable@vger.kernel.org> # v5.9+
> Reported-and-tested-by: piotr.oniszczuk@gmail.com
> Cc: piotr.oniszczuk@gmail.com
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/drm_gem.c              | 4 ++--
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++++-
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 1da67d34e55d..d586068f5509 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1076,6 +1076,8 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
>  	 */
>  	drm_gem_object_get(obj);
>  
> +	vma->vm_private_data = obj;
> +
>  	if (obj->funcs->mmap) {
>  		ret = obj->funcs->mmap(obj, vma);
>  		if (ret) {
> @@ -1096,8 +1098,6 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
>  		vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>  	}
>  
> -	vma->vm_private_data = obj;
> -
>  	return 0;
>  }
>  EXPORT_SYMBOL(drm_gem_mmap_obj);
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index fb11df7aced5..8233bda4692f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -598,8 +598,13 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>  	/* Remove the fake offset */
>  	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
>  
> -	if (obj->import_attach)
> +	if (obj->import_attach) {
> +		/* Drop the reference drm_gem_mmap_obj() acquired.*/
> +		drm_gem_object_put(obj);
> +		vma->vm_private_data = NULL;
> +
>  		return dma_buf_mmap(obj->dma_buf, vma, 0);
> +	}
>  
>  	shmem = to_drm_gem_shmem_obj(obj);
>  

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